Problem: Luis ate 5 slices of cake. Christopher ate 4 slices. If there were initially 13 slices, what fraction of the cake was eaten?
Solution: $\text{fraction of cake eaten} = \dfrac{\text{number of slices eaten}}{\text{number of slices total}}$ They ate ${9}$ out of $13$ slices. They ate $\dfrac{9}{13}$ of the cake.